1. Technical Field
In its general aspect the present invention relates to an urea production high efficiency, low investment synthesis loop based on a CO2 stripping scheme with horizontal lay-out.
2. Prior Art
Most of the today new urea plants are based on the so called xe2x80x9cstrippingxe2x80x9d technologies, namely CO2 stripping by Stamicarbon and Toyo and ammonia xe2x80x9cself strippingxe2x80x9d by Snamprogetti.
All above technologies are characterised by a practically isobaric loop where the main part of unreacted residual CO2 and ammonia contained in the urea reactor effluent are removed in a first carbamate decomposer (stripper), which vapours effluents are at least partially condensed, generating steam in a carbamate condenser. Reactor, stripper and carbamate condenser being the main components of the isobaric loop.
In all above schemes the urea solution effluent from the xe2x80x9cstripperxe2x80x9d is processed in a downstream urea recovery unit to totally remove the residual content of ammonia and CO2 from urea water solution, forming a water carbamate solution which is recycled to the loop. According to the Snamprogetti and Toyo schemes, a separate stream of liquid ammonia, besides the carbamate solution, is also recycled to the loop.
All above schemes operate with high content of water in the reactor, since most of the water contained in the carbamate water solution recycled from the downstream the loop urea recovery unit is fed to the reaction space (reactor). Such high water content (H2O/CO2 mole ratio at reactor inlet: 0.5-0.8) prevent to obtain high intrinsic CO2 conversion yields which are essential to minimise the size of equipment and therefore investments and energy consumption for low operating costs. High intrinsic CO2 conversion yields can be obtained only with low water content such as 0.1-0.2 H2O/CO2 mole ratio at reactor inlet.
In addition, in all above schemes (except the Snamprogetti one and improved Toyo CO2 stripping processes) most of the inert gas introduced in the loopxe2x80x94such as for instance air, hydrogen, etc. the main part of which is contained in the CO2 feed (air is generally added for its oxygen content which is used to passivate and protect against corrosion the loop equipment)xe2x80x94reaches the reactor with negative effect on CO2 conversion efficiency.
Key features of the just mentioned technologies are
A) The CO2 stripping processes (namely Stamicarbon and Toyo): (a1) gravity circulation of vapours and liquid streams in the loop, leading to a xe2x80x9cvertical lay-outxe2x80x9d to secure the above by gravity circulation; (a2) all CO2 feed is sent to the stripper and then to the reactor through the carbamate condenser where vapours are only partially condensed in order to secure the reactor thermal balance; (a3) high water content (high H2O/CO2 ratio) in the reactor since most of the water contained in the carbamate solution recycle coming from loop downstream urea recovery unit, is sent to the reactor through the loop carbamate condenser.
Modifications in the CO2 stripping processes have been recently introduced by Stamicarbon (AIChE Ammonia Safety Symposium, Boston September 1996) and by Toyo (U.S. Pat. No. 5,936,122), both aiming to achieve horizontal lay-outs to reduce investments and for easier maintenance and operation.
Above modified schemes can be summarized as follow:
Modified Stamicarbon CO2 stripping process (FIG. 1 according to AIChE Boston 1996 publication):xe2x80x94use of an horizontal xe2x80x9cpool reactorxe2x80x9d where the functions of carbamate condensation with steam generation and reaction to form urea are combined in one single horizontal equipment, being the circulation of liquid and vapour streams from xe2x80x9cpool reactorxe2x80x9d to stripper still by gravity. The horizontal xe2x80x9cpool reactorxe2x80x9d is located at a higher level above the top of the stripper to secure the gravity circulation of the loop streams.
Modified Toyo CO2 stripping process (FIG. 2 according to U.S. Pat. No. 5,936,122): use of a vertical xe2x80x9cpool condenserxe2x80x9d where the vapours coming from CO2 stripper are totally condensed with steam generation and beginning of urea formation. The carbamate solution from carbamate condenser is recycled to the reactor by the use of an ejector with liquid ammonia as driving fluid. Only part of the CO2 feed is sent to the stripper since part of it has to be sent to the reactor to secure the reactor thermal balance.
B) The Snamprogetti process (FIG. 3 according to UK patent 1188051): (b1) no external stripping agent is used in the stripper, being reactants, including CO2, sent directly to the reactor; (b2) vapours from the stripper are totally condensed in the loop carbamate condenser; (b3) carbamate solution from carbamate condenser recycled to the reactor by the use of an ejector with liquid ammonia used as driving fluid; (b4) high free ammonia residual content in the urea solution effluent from stripper; (b5) high water content (high H2O/CO2 mole ratio) in the reactor since most of the water contained in the carbamate solution recycle coming from loop downstream urea recovery unit, is sent to the reactor through the loop carbamate condenser; (b6) xe2x80x9chorizontal lay-outxe2x80x9d where all loop main equipment (reactor stripper and carbamate condenser) are located at ground level.
The following draw-backs are present in the above described loop schemes:
The improved Stamicarbon CO2 stripping process still has a high water content (high H2O/CO2 mole ratio) in the reactor since most of the water contained in the carbamate solution recycle coming from the loop downstream urea recovery unit is sent to the reacting zone through the carbamate condensation zone of the xe2x80x9cpool reactorxe2x80x9d. That means that the reactor cannot have an intrinsic high efficiency. In addition, the horizontal pool reactor has to be installed at high elevation with high investment costs. High investment costs are also required due to the complexity of the xe2x80x9cpool condenserxe2x80x9d.
In the above schemes high content of inerts are present in the reactor, causing lower CO2 conversion efficiency.
The modified Toyo CO2 stripping process realises the horizontal lay-out but, similarly to the Snamprogetti self stripping process but does not solve the problem of achieving a reaction intrinsic high efficiency with a low H2O/CO2 mole ratio. In addition, the partial use of the CO2 feed to the stripper (a substantial part of the CO2 feed has to be sent to the reactor) significantly reduces the stripping efficiency with higher residual content of unreacted ammonia and CO2 being left in the stripper urea solution effluent to be processed with high investment and operating costs in the loop downstream urea recovery unit. In addition, high investment costs are required due to the complexity of the xe2x80x9cpool condenserxe2x80x9d.
The Snamprogetti process: although it realises an horizontal lay-out, it does not solve the problem of obtaining intrinsic high efficiency with low ammonia H2O/CO2 mole ratio like in the other above mentioned processes. In addition, the Snamprogetti self stripping process, with no use of an external stripping agent, causes a higher residual content of free ammonia in the stripper urea solution effluent with higher investment and operating costs in the downstream urea recovery unit.
The aim of the present invention is to conceive and make available a process and plant related to an urea production high efficiency, low investment synthesis loop based on a CO2 stripping scheme with horizontal lay-outwhich comprises the steps of:
performing a reaction between ammonia (NH3) and carbon dioxide (CO2) in a vertical reaction space (reactor R) to obtain a mixture comprising urea with minimum residual content of unreacted CO2 (maximum CO2 intrinsic conversion yield) in presence of minimum content of water (H2O) and possibly of High Efficiency Trays (HET);
subjecting said reaction mixture to a stripping treatment with the use of CO2 feed as stripping agent in a counter current apparatus (Stripper) to remove most of the unreacted CO2 and NH3 and to obtain a first flow comprising NH3 and CO2 in vapour phase and a second flow comprising urea and residual carbamate and free NH3 in aqueous solution, such residual carbamate and free NH3 being removed downstream the loop in a urea recovery unit, and recycled to the loop in a third flow aqueous solution (carbamate recycle solution: CRS) according to means (urea recovery unit: URU) to separate the urea product in aqueous solution;
subjecting said first flow comprising NH3 and CO2 in vapour phase to total condensation in a carbamate condenser to obtain a carbamate solution which is sent to the reactor space preferably the bottom zone thereof) using an ejector E;
removing the heat formed in NH3 and CO2 total condensation in the carbamate condenser by producing steam preferably also used to preheat the NH3 feed sent to the reactor and the carbamate solution recycle sent to carbamate stripper;
in a preferred embodiment above carbamate condenser is designed to be a vertical heat exchanger and to work xe2x80x9cfull of liquidxe2x80x9d;
separation and removal from carbamate condenser top head of main part of inerts (mainly air introduced through the CO2 to protect equipment against corrosion) contained in the vapours effluent from the stripper;
subjecting at least part of said third flow (carbamate recycle solution CRS) to a treatment (carbamate stripper) for carbamate decomposition and removal of free ammonia to obtain a vapour flow containing CO2 and NH3 with minimum content of water, and a weak water carbamate solution sent back to the urea recovery unit URU;
in a preferred embodiment above carbamate stripper is designed to be a counter current vertical heat exchanger with trays in the upper head;
in a preferred embodiment above carbamate recycle solution CRS is preheated, before being sent to the carbamate stripper, with steam, preferably with the steam generated in the carbamate condenser;
feeding the above vapour flow containing CO2 and NH3 with minimum content of water directly to the loop and preferably to the bottom zone of the reactor space;
feeding at least part of the NH3 feed through ejector to said reaction bottom space together with carbamate solution stream from carbamate condenser;
in a preferred embodiment the above NH3 feed is preheated with steam, preferably with the steam generated in the above condenser before being sent to the reactor;
feeding reactor effluent from upper part of top reaction space to stripping treatment with the use of most, preferably total amount of CO2 feed.
Advantageously, according to the invention, all above-mentioned loop components operate practically isobarically with the following features:
the vapour flow coming flow the carbamate stripper (preferably of counter current vertical type with trays in upper head), containing CO2 and NH3 with minimum content of water, fed directly to the reactor, secure the heat balance of the same without the need to use substantial part of the CO2 feed to be sent to the reactor to sustain the reactor heat balance;
most CO2 feed (more than 90%) can be used in the stripper for an efficient CO2 stripping of urea solution effluent from reactor;
maximum CO2 intrinsic conversion yield in presence of minimum content of water (H2O) and High Efficiency Trays in the reactor;
maximum efficiency of the stripper even if not all CO2 feed is sent to the stripper as a consequence of the high intrinsic conversion yield and minimum content of water. Stripping efficiencies as good or better than the one of the most efficient conventional CO2 stripper can be obtained even if not all the CO2 feed is sent to the stripper;
the total condensation in the carbamate condenser (preferably of xe2x80x9cfull of liquidxe2x80x9d vertical shape) of the vapours comprising NH3 and CO2 coming from the CO2 stripper, to obtain a carbamate solution, significantly increase the carbamate solution residence time in the condenser with formation of urea which contribute to increase the total reaction zone residence time;
the total condensation in the Carbamate Condenser is obtained removing heat as steam which is conveniently used among other users also to preheat the NH3 feed sent to reactor, and the carbamate solution recycle sent to Carbamate Stripper, to secure minimum energy consumption;
the total condensation of NH3 and CO2 vapours in the carbamate condenser allows the recycle of carbamate solution with the use of ejector driven by at least part of NH3 feed, with all loop main components: carbamate condenser, stripper, reactor and carbamate stripper at ground level (horizontal lay-out);
the total condensation of NH3 and CO2 vapours from the stripper in the carbamate condenser allows the separation and removal from the loop (vent) of the main part of inerts (mainly air introduced through the CO2 to protect equipment against corrosion) contained in the vapours effluent from the stripper. In this way the content of inerts in the reactor is minimised with consequently better CO2 conversion efficiency.